Merlin’s Magic Sleepsuit size

So my 3 month old is about 17 lbs. The sleep suit size 3-6 is for lbs 12-18 and the 6-9 mo is for lbs 18-21. I’m not sure what size to get because i can’t find a length for either size. He hasn’t been measured in a while but in carters pjs size 6 mo his feet don’t yet reach the feet of the pjs but he’s too long for 3 mo size. Any recommendations would be greatly appreciated!

I know they say their feet should fit outside of the Sleepsuit so the baby can’t pull their legs inside.